Catalytic sensitization of substrates for metallization

ABSTRACT

Metal films of improved uniformity are formed on substrates having their surfaces sensitized by the deposition of palladium on tin by buffering the palladium salt solution in contact with the glass at a pH from 6 to 9. Buffering is preferably accomplished by contacting the substrate with an aqueous buffering solution and an acidic palladium salt solution.

United States Patent Franz et al.

[451 Mar. 19, 1974 Filed:

CATALYTIC SENSITIZATION OF SUBSTRATES FOR METALLIZATION Inventors:Helmut Franz, Oakmont; Roy G.

Crissman, Lower Burrell, both of Pa.

Assignee: PPG Industries, Inc., Pittsburgh, Pa.

May 28, 1971 Appl. No.2 147,790

U.S. Cl 117/47 A, 106/1, 117/54, 117/130 E, 117/160 R Int. Cl B4411l/08, C030 17/10 Field of Search 117/47 A, 160 R, 130 E, 117/54; 106/1References Cited UNITED STATES PATENTS 12/1968 Sergienko 106/1 3,672,9396/1972 Miller ll7/l30 E 3,370,974 2/1968 Hepfer ll7/l60 R 3,266,9292/1966 Lareau et a1.... ll7/13O E 3,296,012 1/1967 Stalnecker 117/47 A3,438,798 4/1969 Baudrand 11 7 47 A 3,403,035 9/1968 Schneble 106/1Primary ExaminerWilliam D. Martin Assistant Examiner-William R. TrenorAttorney, Agent, or Firm-E. Kears Pollock [5 7] ABSTRACT 5 Claims, 2Drawing Figures INVENTOR] #51 Ml T FRANZ f r a. ems/14AM NON 2N Q @NYNmom M $0M PATENTEDMAR 19 1974 oom ATTORNEY! CATALYTIC SENSITIZATHON OFSUBSTRATES FOR METALLllZATllON CROSS-REFERENCES TO RELATED APPLICATIONSThis invention is related to electroless metal deposition and to theprocesses disclosed in co-pending applications here listed: TransparentMetal-Boron Coated Glass Articles, Ser. No. 57,575; Wet Chemical Methodof Producing Transparent Metal Films, Ser. No. 57,451; Solution forDepositing Transparent Metal Films, Ser. No. 57,754; and Wet ChemicalMethod of Producing Transparent Metal Films, Ser. No. 57,527, all filedJuly 23, 1970. Application Ser. No. 57,575 is now abandoned; applicationSer. No. 57,754 is now U.S. Pat. No. 3,674,517 issued July 4, 1972;application Ser. No. 57,451 is now U.S. Pat. No. 3,723,158 issued Mar.27, 1972, and application Ser. No. 57,527 is now US. Pat. No. 3,723,155issued Mar. 27, 1972.

BACKGROUND OF THE INVENTION This invention relates to chemical platingreferred to in the art as electroless plating, and more particularly, itrelates to a new method ofsensitizing a substrate surface to produce acatalytic surface receptive to the deposition of metal-boron containingfilm.

In the art of metallizing substrates, particularly nonconductivematerials such as non-metals, for example glass and plastics, it hasbeen found desirable to prepare the substrate surface to make it morereceptive to metal deposition.

Bergstrom in US. Pat. No. 2,702,253 teaches an effective method ofsensitizing a substrate surface and thereafter metallizing it utilizingelectroless plating solution. In the method taught by Bergstrom thesubstrate is first treated with a tin salt (a step now referred to inthe art as sensitization), then treated with a palladium salt (a stepnow. referred to as supersensitization) and finally treated with thesalt of the metal desired to be deposited in a suitable electrolessplating bath.

While the method of Bergstrom has been widely accepted by workers in theart, the method has not been free of problems. When depositingrelatively thick films on substrates by immersing the substrates in aseries of baths according to the teachings of Bergstrom, it has beenpossible to produce satisfactory films. However, attempts using themethod of Bergstrom to deposit relatively thin transparent films,particularly by economical and rapid spray techniques suited for theproduction of transparent metallized articles in commercial quantities,have resulted in imperfect films. There has been a tendency of the filmsto be streaked, to have many fine pinholes through the film, to havebands of thicker and thinner film as visually observed and to have athinner film along the leading edge of a moving substrate passing undersprays for contacting the substrate with chemical filming solutions.

SUMMARY OF THE INVENTION lt has been discovered that by buffering apalladium salt super-sensitizing solution in contact with a substrate tobe sensitized for chemical metal film deposition to a pH between about 4and about 9 that uniform metal films may be obtained when thin,transparent metal films are subsequently deposited on the sensitizedsurface from electroless plating baths. The presence of a colloidalsuspension of palladium hydroxide in contact with the substrate beingtreated enhances the uniformity of subsequently deposited films. A pHwithin the range of from 6 to 9 for the palladium solution in contactwith the substrate provides assurance of this desired condition forpalladium treatment.

A suitable process of metallizing a plurality of substrates, such asflat glass articles, is to convey the articles in series past severalspray, drip or flood contacting stations. The process includes: rinsingthe glass, contacting the glass with a tin salt solution, rinsing awaythe residue of tin salt solution, contacting the glass with a palladiumsalt solution, rinsing away the residue of the palladium salt solutionand contacting the glass with one or more sets of metal salt solutionsand boroncontaining reducing solutions followed by rinsing, drying andinspection of the metallized glass. By including a suitable buffer, suchas sodium bicarbonate, in a rinse between the tin salt solution and thepalladium salt solution, residue buffer is retained on the glass tobuffer the pH of the contacting palladium salt solution to a pH of from4 to 9 and preferably from about 7 to about 9.

Other buffering agents may be successfully used rather than sodiumbicarbonate. Typical, commercially available buffers which may be usedinclude water soluble alkali metal carbonates, alkali metalbicarbonates, alkali metal phosphates, ammonium hydroxide, ammoniumcarbonate, ammonium bicarbonate, ammonium phosphate and ammonium borate.The dibasic phosphates are preferred as contrasted with otherphosphates. Alkali metal salts of organic acids may be effectivelyemployed as well; sodium acetate, for example, is effective to controluniform supersensitization. Also, mixtures of buffers, such asphosphates, and alkali metal hydroxides are effective buffers in thisinvention.

Although other tin and palladium salts may be used, the halides arepreferred. Stannous chloride (SnCl is the preferred tin salt for initialsubstrate surface sensitization. Palladious chloride (PdCl is thepreferred palladium salt for supersensitization of the substratesurface, although palladic chloride (PdCl may be suitably employed.

It is preferred that water used for preparing the aqueous sensitizingand chemical filming solutions and the water for rinsing the substratebe softened, deionized or otherwise purified since in the commercialproduction of metallized articles film quality may be detrimentallyaffected by the uncontrolled chemical composition of process waterentering a plant from public water supplies. Throughout the presentdiscussion, use of the words water, demineralized water or the likeshall mean water substantially free of organic and inorganiccontaminants, for example an indicated resistance of more than onemegaohm/cm at 25C. would indicate pure water. Throughout words, such astap water, shall mean water of unknown quality as typically obtainablefrom public supplies.

it is preferred that the palladium chloride contacting solution has a pHfrom about 1 to 4, preferably from about 2 to 3.9, and most preferablyfrom about 3.0 to 3.6.

The buffer may be included in the intermediate rinse between tinsensitization and palladium supersensitization, or the substrate may berinsed with water and then contacted, as by a spray or drip, with anaqueous buffer solution immediately prior to the palladium salt solutioncontacting the substrate. in the latter embodiment buffer solutions of0.1 to 70.0, preferably 0.5 to 5 and most preferably 0.5 to 1.5 grams ofbuffer per liter of water, when dripped on a horizontally disposed glasssheet being conveyed under spray contacting stations, suitably bufferthe immediately following palladium salt solution in contact with theglass.

The present discovery provides a method by which more uniform,mettle-free, metal-containing films may be deposited on a catalyzednon-metallic substrate than has been possible before. Bands ofnon-uniform film thickness, streaks and pinholes through the depositedmetal-containing films are substantially reduced and commonly are absentfrom films produced by the method of this invention. Thinning out of thefilm toward the leading edge of substrates metallized while moving pastcontacting stations is substantially reduced and generally eliminated bythis method. It has also been discovered that the adhesion of thedeposited film to the glass or other substrate is improved by applyingfilm by this method. This results in films of improved durability whenthe films are applied according to this invention.

Any metal suited for chemical filming on a catalytic sensitizedsubstrate may be deposited according to the method of this invention.Chromium, manganese and the metals of Groups VIII and [B of the PeriodicTable may be applied to substrates according to this invention. Ofparticular interest are the lightest elements of Group VIII, iron,cobalt and nickel. The method of this invention is particularly suitedfor the application of iron, cobalt or nickel to substrates whichrequire sensitization, such as glass.

The method has particular utility for producing thin, transparentmetal-containing films on transparent substrates, for example glass orplastics, such as polycarbonate, acrylics and the like. The desirabilityof uniform films is particularly critical for transparent viewingclosures. Visible transmittance of flat, clear, limesoda-silica glassmetallized using the present method may be as low as 8 percent and yetbe uniform. Transparency is determined as the percent of light having awave length from 380 to 760 nanometers which is transmitted through thecoated article relative to the transmittance through air as measured bya Beckman Model DK 2A Spectrometer.

The mechanism by which the buffering of palladium salt sensitizationsolutions promotes film uniformity is not fully understood. In fact, themechanism by which palladium catalytically sensitizes substrates is notunderstood with universal agreement among those skilled in the art. Thislack of agreement and understanding persists despite the many years ofusing palladium sensitization.

Despite a lack of full understanding as to why the present invention iseffective, the following observations may be made. Palladium chloridesolubility in water is influenced by pH. High concentrations ofpalladium chloride in a sensitizing solution improve sensitization ofthe surface, presumably by increasing the density of catalytic sites atthe sensitized substrate surface. The preferred pH range of a suitablyconcentrated palladium chloride solution is from 1 to 4 with higherconcentrations attainable at lower pH. At a pH of 3.9 some incipientPd(OH) precipitation is noted.

A colloidal suspension apparently is present upon adjusting pH from 6 to9 as indicated by a yellow coloration typical of colloidal suspensions;at pH 6 to 7 the characteristics of colloidal suspension are apparent;at pH 7 to 9 a colloidal suspension remains with the apparent particlesize of the precipitate smaller at pH 8 than at pH 7.8. It appears thatprecipitation and formation of a relatively stable colloidal suspensionof palladium hydroxide in the supersensitization solution whencontacting the substrate is at least partially responsible for thebeneficial results obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS through the section. Insections 200-500, the apparatus comprises a plurality of rollers 2 forcarrying and advancing the plates through the sections. The belts androllers are rotated by conventional means (not shown) to advance theplates at about 3 to 6 feet per minute.

In operation, plates 3 are loaded onto the belts 1 and advanced throughsection 100. In this section, four rotating blocks I01 comprised ofbrushes gently abrade the upper surface of the plates 3 with a mixtureof amorphous silica and water or cerium oxide and water to loosen andremove any dirt. The blocks are rotated on shafts 102 at a rate of about350 rpm and are oscillated in a direction transverse to the motion ofthe advancing plates at a frequency of about 15 cycles per minute and anamplitude of about 4 inches. Four rotary cup brushes 104 are arranged on12-inch centers in a line traversing the belt and roller conveyororientation such that the longitudinal distance between the blocks 101and the rotary cup brushes is about 9 inches. The rotary cup brushes arerotated at about 350 rpm and oscillated in a transverse direction with afrequency of about 15 cycles per minute and an amplitude of about 4inches. A spray of water, the purity of which is not critical, is usedto remove residual silica or cerium oxide. A rotary cylinder brush 105is disposed transversely across the conveyor and is rotated to wipe awayexcess water and residue silica or cerium oxide.

After cleaning, the plates are advanced into section 200 for surfacesensitization. As the plates pass through section 200, each is firstrinsed with water which is substantially pure or demineralized. Therinse is accomplished by employing a cross-fire spray. A mutuallyopposed pair of spray guns, 201 and 202, are supported from a carriage203 and reciprocated transversely of the plates on a track 204 at a rateof about 40 to 60 cycles per minute. During the reciprocation ofcarriage 203, rinse water is fed to guns 201 and 202 in alternatingintermittent fashion such that water is sprayed only from gun 201 whenthe carriage moves in one direction (from bottom to top of FIG. I), andwater is sprayed only from gun 202 when the carriage is moving in theopposite direction. The guns 201 and 202 are tilted slightly toward eachother to provide a cross-fire effect or sweeping action tending to washexcess water from the surface of the plates.

After undergoing the initial rinse with demineralized water, each plateis advanced beneath a spray gun 211 which is mounted on thereciprocating carriage 203. A stannous chloride solution (a preferredcomposition is given below) is fed to spray gun 211 which directs thesolution against the top surface of each advancing plate.

Each plate then advances beneath an intermediate set of cross-fire rinseguns 212 and 213. These guns are similar to guns 201 and 202 instructure, mounting and operation. Water containing a sufficientbuffering agent, preferably sodium bicarbonate, to buffer the laterapplied palladium supersensitizing solution is fed to guns 212 and 213and onto the top surface of each plate. Alternatively, a drip applicator218 comprising a pipe having a plurality of orifices disposed along itslength is positioned between guns 212 and 213 and spray gun 214 and abuffering solution is applied to each plate through such an applicator218 following rinsing with demineralized water from guns 212 and 213.

In any event, buffer-containing rinse water is retained on each plate asit advances beneath spray gun 214. Spray gun 214 is also mounted oncarriage 203 to reciprocate across each plate. A palladium chloridesolution (a preferred composition is given below) is fed to spray gun214 which directs the solution against the top surface of each advancingplate. The palladium chloride solution, as it is sprayed from the gun214, is acidic, generally pH 3.9 or less, in order to maintain a highconcentration of palladium chloride in solution. The solution isatomized exiting the spray gun 214 and contacts the plate below mixinginto the residue rinse water containing an effective amount of buffer toadjust the pH of the mixed solutions on the plate to at least 4 andpreferably to pH 6 to 9, preferably about pH 8.

Each plate then advances past cross-fire rinse spray guns 215 and 216,which are similar in construction, mounting and operation to cross-firespray guns 201 and 202 and 212 and 213. Demincralized water is fed toguns 215 and 216 though the water quality is less critical than therinse prior to palladium supersensitization.

As illustrated, the first, second and third cross-fire rinse guns, aswell as the tin gun and the palladium gun, i.e., all of the guns insection 200, are mounted from a single boom that reciprocates in thetransverse direction at a rate of about 54 single passes per minute.Each of the rinse guns comprises a single UniJet-TSOOI or T8002 spraynozzle (Spraying Systems Co., Bellwood, lll.) operated at a pressure ofabout 40 psig. and an average flow rate of about 0.12 to 0.20 gallon ofrinse water per minute. Each of the tin and palladium guns comprises asingle type C spray gun equipped with a Paasche U2, F2-8 nozzle (PaascheAir Brush Co., Chicago, lll.) operated at an air pressure of about 30 to70 psig. and at a flow rate of about 800 milliliters of the solutiondescribed below per minute. The distance between the rotary cylinderbrush and the first cross-fire rinse guns 201 and 202 is 36 inches,while the distance from each gun or gun set in section 200 to the nextrespective gun or gun set is l8 inches.

Each plate is then advanced through section 300, wherein a metal-boroncontaining coating is deposited on the now catalytically activated orsensitized surface of each plate by simultaneously spraying andintermixing a metal-containing solution and a boron-containing reducingsolution onto the activated surface such that the metal ions present inthe contemplated metal solution became reduced to a transparentboroncontaining metal film which tenaciously adheres to the activatedsurface. For the sake of illustration, section 300 is shown to have fourgun sets 301-304 each comprising a metal-containing solution gun and amutually opposed reducing solution gun. Section 300 also includes amutually opposed pair of water spray guns 305 and 306 arranged forcross-fire rinsing. As shown, the guns 301304 are supported fortransverse reciprocating movement in the manner described above.However, it should be noted that the gun sets in section 300 aregenerally reciprocated in the transverse direction at a rate greaterthan the reciprocation rate in section 200, for example, a rate of 74single passes per minute. During operation, each of the metal depositiongun sets in section 300 is maintained at a pressure of about 40 psig.and a flow rate of about 800 milliliters of solution per minute, whilethe final cross-fire rinse guns 305-306 are operated at a pressure ofabout 40 psig. and an average flow rate of about 0.12 to 0.20 gallon ofrinse water per minute.

The gun sets in section 300 of the apparatus are spaced apart from thosein section 200 such that the distance between the last rinse guns insection 200 and gun set 301 is about 54 inches. In addition, gun set 301is spaced apart from gun set 302 such that the sprays generated fromthese sets (301-302) are overlapped, and such that the residence time ofeach plate in the spray area defined by gun sets 301 and 302 (spray areaI) provides for deposition of a controlled film thickness. The residencetime in the dwell area between gun set 302 and gun set 304 (dwell area1), the residence time in the spray area of gun set 304 (spray area ll)and the residence time in the dwell area between gun set 304 and rinseguns 305-306 (dwell area ll) also are established to control filmthickness. Gun set 303 may optionally be used to apply additional metalsalt and reducer solution. The metal-reducer gun sets (301-304)typically employed have Paasche U2, F2-8 nozzles, while the rinse guns305-306 each comprise a single UniJet-T800l or T8002 spray nozzle.

After undergoing a final water rinse under guns 305 and 306, each plateadvances into section 400 of the coating apparatus where it is driedwith an air knife 401 comprising an elongated metal housing having an0.002 inch delivery channel extending along the length thereof. Theknife 401 is disposed at a 45 angle relative to the advancing plate andhas its centermost portion spaced about 48 inches from the final rinseguns. The air knife is operated at about 5 psig. and an air flow rate ofabout 4,000 to 6,000 cfm. After passing beneath the air knife, eachplate passes beneath a Gardner- Large Area Hazemeter 501 which measuresand records the luminous transmittance of the coated plates.

The ambient air temperature during film deposition is about Fahrenheit,while the temperature of the demineralized and tap water used throughoutthese examples is generally about 65 Fahrenheit. The temperature of themetal and reducer solutions is about 80 Fahrenheit. On the basis of aliter of solution, each of the prepared aqueous solutions employed hadthe following composition:

NICKEL SOLUTION PREFERRED RANGE Nickelous acetate 5 grams 4-l0 gramsBoric acid 2.5 grams 2-5 grams Sodium gluconate 9.0 grams 7-18 gramsHydrazine sulfate 0.5 gram 0.4l.0 grams Water added to 1 liter Ammoniumhydroxide added to pH 7.2 pH 7.0-7.6 Ethomeen C-20 1 drop per liter ofsolution 0-2 drops Acetone 0.0! gram 0-100 grams 'Ethomeen C-20(trademark of Armour and Company) is a coeoamine having an averagemolecular weight of 645 and the following generalized formula:

The pH of the intermixed nickel and borohydride solutions is about 7.7.

The amount of buffered rinse water sprayed onto each plate is adjustedso that the pH of the mixture of the palladium solution and bufferedrinse on the plate is about pH 8.

Table 1 summarizes a series of metallizing runs following the proceduredescribed above, except as indicated.

TABLE I Run Rinse After pH After Luminous No. Tin Solution PdCl SprayTransmittance Film Quality l Demineralized 3.7 20 percent Mottled.

Water leading portion of plate with less film TABLE l-Continued RunRinse After pH After Luminous No, Tin Solution PdCl, Spray*Transmittance" Film Quality 2 Buffered 7.8 20 percent Smooth,

Water l uniform fine gm/liter, grain NaHCO;)

3 Demineralized 8.0 20 percent Smooth,

Water uniform fine followed by grain drip with l gram/liter 4)2 a" H Oin water 4 Buffered 20 percent Smooth,

Water (0.25 uniform fine gm/liter grain. but NaHCO leading portionsomewhat lighter in film Estimated by one twenty-fifth dilution of PdCl,spray solution. Other rinses in the system were varied as dernineralizedwater and buffered water without effect. "Determined using Beckman DK 2ASpectrometer for visible light, 380-760nm.

Sodium carbonate is substituted for the sodium bicarbonate of run 2 inan amount of 1 gram per liter of rinse solution. The resulting pH isestimated as l 1.9. The leading edge film deficiency is eliminated aswith sodium bicarbonate. However, the film texture is more coarselytextured than when sodium bicarbonate is used.

Substitution of sodium acetate in an amount of 2 grams per literimproves film uniformity and texture but not as markedly as does sodiumbicarbonate.

Ammonium bicarbonate, dibasic sodium phosphate, ammonium borate and acommercially available mixed buffer of monobasic sodium phosphate andsodium hydroxide (Fisher Scientific pH Seven") are each used in anamount of one gram per liter of rinse as in run 3. Each buffereliminates a leading edge film deficiency, each improves texture of thefilm and all but the borate result in an extremely fine grain film.

While the present invention is described with particular reference tospecific embodiments, it is not intended to be limited by specificchemicals or means of application of buffer in the preparation of thesubstrate to be coated.

We claim:

1. In the electroless deposition of metal on a nonconductive substratewherein a substrate is contacted successively with a tin-salt solution,then with a palladium salt solution to activate it, making its surfacecatalytic, and then is contacted with a mixture comprising a solution ofmetal salt of the metal to be deposited and a solution of a reducingagent for reducing such metal, the improvement comprising buffering thepalladium salt solution in contact with the substrate at a pH from about7 to about 9.

2. [n the method of coating a non-conductive substrate surface with ametal-containing deposit compris ing the steps of:

first, contacting the substrate with a solution of tin salt and thenrinsing the same;

second, contacting the substrate with a solution of palladium salt andthereafter contacting the substrate with a solution containing thedesired metal to be deposited and reducing agent for such metal, wherebya non-uniform metal-containing deposit is formed, the improvementcomprising the steps of:

rinsing the tin salt contacted substrate with a rinse of watercontaining an alkaline buffering compound in an effective amount toprovide a pH of about 7 to about 9 in the mixture of palladium saltsolution and remaining rinse in contact with the substrate whereby auniform metal-containing deposit results. 3. The method of claim 2wherein the contacting solutions are sprayed against the substrate.

bonate.

2. In the method of coating a non-conductive substrate surface with ametal-containing deposit comprising the steps of: first, contacting thesubstrate with a solution of tin salt and then rinsing the same; second,contacting the substrate with a solution of palladium salt andthereafter contacting the substrate with a solution containing thedesired metal to be deposited and reducing agent for such metal, wherebya non-uniform metal-containing deposit is formed, the improvementcomprising the steps of: rinsing the tin salt contacted substrate with arinse of water containing an alkaline buffering compound in an effectiveamount to provide a pH of about 7 to about 9 in the mixture of palladiumsalt solution and remaining rinse in contact with the substrate wherebya uniform metal-containing deposit results.
 3. The method of claim 2wherein the contacting solutions are sprayed against the substrate. 4.The method of claim 2 wherein the alkaline buffer is selected from thegroup consisting of ammonium hydroxide, ammonium borate andwater-soluble ammonium, alkali metal and alkaline earth metalcarbonates, bicarbonates and phosphates and mixtures thereof.
 5. Themethod of claim 4 wherein the rinse contains as an esential ingredientfrom about 0.01 to about 7 percent by weight of the aqueous rinse,sodium bicarbonate.